1. Field of the Invention
The present invention relates to an FM modulator and, more specifically to an FM modulator for forming FM carrier capable of precise detection of jitter in reproduced video signals in a recording/reproducing apparatus. More specifically, the present invention relates to a carrier reset FM modulator for resetting phase of the FM carrier to be recorded at every horizontal period and to a method of FM modulating video signals.
2. Description of the Background Art
When video signals are recorded and reproduced by a video tape recorder (video cassette recorder) using a magnetic tape as a recording medium, time axis (base) fluctuation, that is, jitter of the reproduced video signals is generated, derived from fluctuation in tape running system. The jitter causes vibration in the reproduced image and no transmittance of reproduced video signals. Therefore, in reproducing circuitry, it is necessary to correct the time axis by detecting jitter so as to ensure stability of the reproduced image.
In a general video signal recording/reproducing apparatus, a specific portion of a rise or fall of a horizontal synchronizing signal included in the reproduced video signal is detected as a signal for detecting jitter, and correction of jitter is carried out based on the jitter detecting signal.
However, generally noise having random phases and levels is superimposed on the horizontal synchronizing signal. Such noise makes difficult the precise detection of rise/fall of the horizontal synchronizing signal, degrading precision of the jitter detecting signal. The degradation in precision of the jitter detecting signal makes sufficient correction of jitter difficult, and hence makes instable the reproduced image. Various jitter detecting apparatuses for precisely carrying out jitter correction have been proposed. Japanese Patent Laying-Open No. 63-274290, Japanese Utility Model Laying-Open No. 1-82572, Japanese Patent Laying-Open No. 1-264492 and Japanese Patent Laying-Open No. 1-305785 are examples disclosing such prior art.
Japanese Patent Laying-Open No. 63-274290 discloses a method of detecting jitter in which phase of FM carrier corresponding to horizontal synchronizing signal portion of incoming video signal is reset for every horizontal synchronizing signal before recording-, when video signals are recorded, and in reproduction, the reset portion is used as a reference burst signal to provide a jitter detecting signal.
Japanese Utility Model Laying-Open No. 1-82572 discloses an FM modulating circuit in which two carrier reset (resetting of FM carrier) phases are provided in 1 cycle of the FM carrier and these two points are reset to either of the phases having phase difference of 180.degree. whereby ringing pulse-like noise at a fall portion of the horizontal synchronizing signal generated in FM demodulator output during reproduction is reduced, and hence precision in jitter detection is improved.
Japanese Patent Laying-Open No. 1-264492 discloses a jitter detecting apparatus in which a pulse-like signal in white direction is added subsequent to a video signal at a front porch portion immediately before the fall of the horizontal synchronizing signal, and by adjusting pulse width and/or pulse height thereof, ringing at the fall portion of the horizontal synchronizing signal in FM. demodulator output is prevented, and discontinuous and abrupt change of the FM carrier phase derived from carrier reset is suppressed.
Japanese Patent Laying-Open No. 1-305785 discloses a jitter correcting apparatus in which recording is done while carrier reset is carried out with horizontal synchronizing signal of positive polarity of a MUSE (Multiple Subnyquist Encoding) signal being replaced by a negative polarity horizontal synchronizing signal, the carrier reset portion is used as a reference burst signal in reproduction to carry out jitter correction, and then the synchronizing signal of negative polarity is converted into the synchronizing signal of positive polarity.
In the above described prior arts, jitter detecting signals of high precision are formed by making constant the phase of the FM carrier by carrier resetting, so as to stabilize reproduced image through sufficient correction of the jitter.
The reproduced image should be as stable as possible. Therefore, it is desirable to carry out jitter correction with the jitter detecting signal detected with higher precision. However, the following disadvantages arise, when the jitter detecting signals are to be obtained with higher precision.
A preemphasized video signal is applied to an FM modulator. This preemphasis changes the frequency corresponding to a tip of a synchronization signal. When carrier reset is made for resetting a phase of the FM carrier with random phase to 0.degree. or 180.degree., there will be a discontinuation in the phase of the FM carrier, as shown in FIG. 1(B). In FIG. 1, the carrier reset point is shown at a position behind the leading edge of the horizontal synchronizing signal (H-SYNC). If a signal is FM demodulated with the phase of the FM carrier signal being discontinuous, an undershoot is generated in the reproduced FM carrier (FM demodulator output) corresponding to the discontinuation of the phase as shown in FIG. 1(C). If such an undershoot is generated at the leading edge portion of the horizontal synchronizing signal, it causes a distortion at the leading edge portion of the horizontal synchronizing signal (saw-tooth shaped level change). This distortion makes the separation of synchronization signals unstable to cause a gating pulse for detecting a zero-cross point unstable, resulting in degraded precision of the jitter detecting signal, and makes the image instable. Such a distortion of the undershoot causes a distortion at a sync tip, which in turn generates a clamp error, resulting in poor synchronizing signal detection and fluctuation of luminance level of the reproduced video signal after FM demodulation.
An AFC (Automatic Frequency Control) circuit is provided in some FM modulators for FM modulating the incoming video signals. The AFC circuit has a function of comparing the incoming video signal with an AFC reference frequency signal, and of maintaining constant the frequency of FM modulated signal, which is the FM carrier, in accordance with the result of comparison.
The AFC reference frequency signal is generated from a reference frequency generator. The reference frequency generator comprises an oscillating circuit such as a crystal oscillator, whose oscillation characteristic tends to be changed due to temperature change or voltage change. Therefore, the frequency of the AFC reference frequency signal generated from such a reference frequency generator may vary easily as temperature or the voltage is changed. This variation causes fluctuation of the phase of the carrier reset FM carrier, makes unstable the phase of the reference burst signal obtained from the carrier reset FM carrier, causes deviation of the zero cross point, thereby degrades precision of the jitter detecting signal.
As described above, when a jitter detecting signal having higher precision is to be provided, the fluctuation of frequency of the AFC reference frequency signal derived from change in temperature or in voltage must be controlled. In addition, an undershoot generated in the horizontal synchronizing signal after FM demodulation caused by carrier reset must be reduced.